The present invention relates generally to wireless communication systems and, more particularly, to a system and method for reducing interference and increasing the capacity of a cellular network.
Cellular networks have remained relatively static since they were first designed in the late seventies. This original design included simple methods for assigning frequencies by sets and general rules regarding reuse distance, frequency assignments, and antenna orientation.
FIG. 1 is a diagram of a conventional three-sectored cell site deployment 100. Each cell site 110 includes three antennas, for example, antenna 120 pointing north (0xc2x0), antenna 130 pointing southeast (120xc2x0), and antenna 140 pointing southwest (240xc2x0). Ideally, an antenna transmits signals at full power levels within its sector and no signals beyond the cross-over points between sectors. In practice, however, a gradual fall-off results with any angular deviation from the bore sight (i.e., the direction of the antenna). This fall-off has increased in recent years because cellular providers use narrower antennas to reduce interference between antennas in the same cell site.
FIG. 2 is a diagram of power levels of signals transmitted by a typical antenna deployed in a cellular network. In the figure, {circumflex over (1)} identifies the highest power levels along the bore sight and {circumflex over (2)} identifies the decreased power level at the cross-over points between sectors. The power loss at the cross-over points can be as much as 10 dB for very narrow beam antennas.
FIG. 3 is a diagram identifying power levels of the cell sites in the conventional cell site deployment of FIG. 1. FIG. 3 identifies the power levels at the cell edges, using {circumflex over (1)} to identify the highest power level along the bore sight of an antenna and {circumflex over (2)} to identify the decreased power level at the cross-over points between sectors. Location 310 identifies a point of intersection along the bore sights of three antennas from the three closest cell sites. Location 320 identifies a point of intersection of six antennas, two from each of the three closest cell sites.
At points of intersection, such as location 320, six potential signals of equal power levels exist from the three closest cell sites. In the case of Code Division Multiple Access (CDMA) systems, these signals include pilot and associated traffic channels. In the case of analog systems, such as Advanced Mobile Phone Systems (AMPS), these signals include analog channels. At all points, for example location 320, the signals from the different antennas undergo log-normal shadowing, which is the variation of the average power level due to reflections from buildings and other structures. This can result in ping-ponging between the cell sites and increased interference, because a signal not used by a receiver constitutes interference.
FIG. 4 is a diagram showing the effect of interference caused by second tier cell sites in the conventional cell site deployment of FIG. 1. Second tier cells sites include a second ring of cell sites with respect to a point of interest. In FIG. 4, cell sites 410-430 are second tier cell sites with respect to an intersection point at location 320.
Cell sites 410-430 include antennas 415-435, respectively, pointing along their bore sights to location 320. At location 320, the closest cell sites 440-460 transmit signals at low power while the second tier cell sites 410-430 transmit signals at full power (i.e., along the bore sights of their antennas) in this direction. The signals from the second tier cell sites 410-430 result in added interference at location 320.
Typically, the path loss for a conventional cell site deployment, such as the one shown in FIG. 1, is 40 dB per decade. This results in three additional signals at location 320 that have power levels of 12 dB-xcex3 below the power levels of the other signals at this location, where xcex3 refers to the antenna loss at the cross-over point relative to the bore sight gain. Thus, if the antenna pattern drops by 6 dB at a deviation of 120xc2x0 from its bore sight, then a total of nine signals within 6 dB of each other exist at location 320.
In a propagation environment where the path loss exponent is much less than 40 dB per decade (e.g., microcell and other line-of-sight environments), the effect at intersection points, such as location 320, can be even more pronounced. For example, in a line-of-sight environment, at least nine pilot channels of similar power levels could be received at location 320.
A CDMA system configured in a manner consistent with the IS-95 standard can only decode three pilot and associated traffic channels at any one time. The rest of the signals appear as interference. The fading environment and shadowing of the existence of so many signals cause excess overhead messages between the base station and the mobile as the pilots enter and leave the active set. The IS-95B standard will better handle this situation by only reporting those situations where the system requires the adding or dropping of signals from the active set. This standard will not, however, reduce the amount of interference.
In the case of analog systems, the ratio of the channel power to the level of interference (C/I) can be relatively low at intersection points, such as location 320, because of the presence of many interfering signals. This results in the effective reuse of analog channels being larger than necessary and dominated by locations, such as location 320. The end result is the lowering of the cell site capacity because the reuse distance, from location 320""s point of view, becomes higher than necessary.
Therefore, a need exists to reduce the interference resulting from conventional cell site deployment to, thereby, increase the capacity of the cellular network.
Systems and methods consistent with the present invention address this need by reconfiguring the cell sites to increase the capacity of individual sectors and cell sites.
In accordance with the purpose of the invention as embodied and broadly described herein, a system consistent with the present invention includes first cell sites having a first layout and second cell sites, interspersed among the first cell sites, having a second layout. The layout refers to the orientation of the antennas in the cell sites. By making the second layout different than the first layout, the amount of interference in the system reduces and, thus, the capacity of the system increases.